Reimplement apropos -s NUM -S ARCH EXPR by internally converting it to

Reimplement apropos -s NUM -S ARCH EXPR by internally converting it to
apropos \( EXPR \) -a 'sec~^NUM$' -a 'arch~^(ARCH|any)$'
in preparation for removal of sec and arch from the mpage table.

Almost no functional change except for the following bonus:
This also makes sure that for cross-section and cross-arch MLINKs,
all of the following work:
apropos -s 1 encrypt
apropos -s 8 encrypt
apropos -s 1 makekey
apropos -s 8 makekey
Before this commit, they don't, neither for the
old makewhatis(8)/apropos(1) combo nor for the new one.

While here, print error messages about invalid regexps to stderr.