Implement the \\$@ escape sequence (insert all macro arguments,

Implement the \\$@ escape sequence (insert all macro arguments,
quoted) in addition to the already supported \\$* (similar, but
unquoted).  Then use \\$@ to improve the implementation of
the .als request (macro alias).

Needed by groff_hdtbl(7).
Gosh, it feels like the manual pages of the groff package are
exercising every bloody roff(7) feature under the sun.  In the
manual page source code itself, not merely in the implementation
of the used macro packages, that is.