grep: fake GNU behavior for non-POSIX empty regex.

POSIX says there's no such thing as an empty regular expression. The
grammar excludes the possibility:

  https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html

BSD agrees with POSIX, and Android and macOS' BSD-based implementations
reject the empty regular expression.

GNU apparently disagrees.

Luckily, BSD does accept the empty *sub* expression `()`, despite their
error message for REG_EMPTY being "empty (sub)expression". This is
presumably a bug, except there's explicit code to support it that is at
least 26 years old:

  https://github.com/freebsd/freebsd/blame/master/lib/libc/regex/regcomp.c#L383

This workaround also works fine with glibc.

If we want GNU behavior, I'm struggling to come up with another way to
fake it. If we want POSIX behavior, we could easily just add a check to
reject "" on glibc.

Also switch to xregcomp().

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