Fix some regressions with the zgrep(1) wrapper.

Fix some regressions with the zgrep(1) wrapper.

 - Handle whitespace with long flags that take arguments:

	echo 'foo bar' > test
	zgrep --regexp='foo bar' test

 - Do not hang reading from stdin with patterns in a file:

	echo foobar > test
	echo foo > pattern
	zgrep -f pattern test
	zgrep --file=pattern test

 - Handle any flags after -e:

	echo foobar > test
	zgrep -e foo --ignore-case < test

These two are still outstanding problems:

 - Does not handle flags that take an argument if there is no
   whitespace:

	zgrep -enfs /etc/rpc

 - When more than one -e pattern used matching should occur for all
   patterns (similar to multiple patterns supplied with -f file).
   Instead only the last pattern is used for matching:

	zgrep -e rex -e nfs /etc/rpc

   (This problem is masked in the unpatched version by the "any
   flags after -e" problem.)

Add tests for the above problems.

Update the mange and add references to gzip(1) and zstd(1) and also
document the remaining known problems.

PR:		247126
Approved by:	markj
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D25613