Fixed some incorrect coloring of broken links when links are being followed (-L)

Made the tests work when run as root by dropping privileges. This may be helpful for certain packaging or CI environments, but is not recommended.

Treat empty PAGER and LESS environment variables like they're unset, for bfs -help (#71). Thanks @markus-oberhumer!

The soft RLIMIT_NOFILE is now raised automatically to a fairly large value when possible. This provides a minor performance benefit for large directory trees.

Implemented time units for -mtime as found in FreeBSD find (#75)