Teach Configure about "procselfexe" on Solaris and NetBSD

Teach Configure about "procselfexe" on Solaris and NetBSD

Configure would already find /proc/self/exe on NetBSD, where /proc/self is
a symlink to /proc/curproc. However, the revised probe avoids the extra
symlink traversal. Configure did not previously probe for the
relevant path on Solaris, /proc/self/path/a.out

Rename the description of /proc/curproc/file from BSD to FreeBSD, as it seems
that of the "big 3" BSDs, only FreeBSD uses this path.

Based on a patch from Johann 'Myrkraverk' Oskarsson.