i#3862: parse /etc/ld.so.conf

This addresses issue #3862 (originally #460 (closed)) by implementing code that parses the search paths in /etc/ld.so.conf for the private loader on Linux.

The /etc/ld.so.conf file consists of one or more lines, where a line may be a search path, an include directive to include another configuration file or where a line may start with '#' if it is a comment. The include directives support globbing (see man 3 glob and man 7 glob for more information).

Some notes about compatibility:

• Alpine is a musl-based distribution, and the dynamic linker used by Alpine instead may look for/etc/ld-musl-$(ARCH).path. For instance, on x86-64, it would look for /etc/ld-musl-x86_64.path. The format seems to be slightly different from /etc/ld.so.conf. If the file is not present, musl simply defaults to "/lib:/usr/local/lib:/usr/lib". See also: https://www.musl-libc.org/doc/1.0.0/manual.html
• Android uses bionic as its libc and instead looks for /system/etc/ld.config.txt. See also: https://android.googlesource.com/platform/bionic/+/master/linker/ld.config.format.md
• While the FreeBSD man pages do seem to mention the existence of /etc/ld.so.conf, the default seems to be that the dynamic linker on FreeBSD looks for the ldconfig_paths and ldconfig32_paths keys in /etc/defaults/rc.conf.
• Mac OS X does not seem to have /etc/ld.so.conf either, but man dyld seems to provide some useful hints.

We probably want to open new issues referring to #3862 and #460 (closed) for the other cases.

This PR implements the following:

• The strncasecmp function to match the include directive. On second thought, strncmp may work well enough, but it doesn't hurt to be case-insensitive.
• The fnmatch function, which is required to implement glob. The fnmatch lets us match a given string against a pattern including character ranges (e.g. [a-z]), the any character (denoted as ?) and wildcards (denoted as '*'). Furthermore, multiple wildcards may be present.
• The glob function which uses fnmatch function and the Unix-specific directory iterator code that is currently present in DynamoRIO to match all found filenames against a given pattern.
• The getdelim and getline functions which help us parse the config files line-by-line.
• The privload_parse_ld_conf function which parses /etc/ld.so.conf and any included config files and constructs an array of search paths by piggybacking on the glob function, which can be used to append search paths to an array of search paths.
• Finally, the privload_locate function has been extended to use the privload_parse_ld_conf, and otherwise falls back to the fixed array of system paths (see compatibility notes).

While the ld.so.conf parsing itself is straight-forward on any POSIX system due to the existence of glob and getline, DynamoRIO originally did not provide these functions as the private loader cannot rely on libc, and it would be helpful to have a thorough review of the implementation of these functions.

I have tested this on:

• My desktop running Gentoo where libgcc_s.so.1 is located in /usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0 by running: ./bin64/drrun -c ./api/bin/libempty.so -- ls. Before these changes, it was necessary to specify the correct path using the LD_LIBRARY_PATH environment variable.
• Ubuntu 20.04 LTS by running: ./bin64/drrun -c /api/bin/libempty.so -- ls to be sure that there are no regressions introduced by this PR.