Rename mandoc_getarg() to roff_getarg() and pass it the roff parser
struct as an argument such that after copy-in, it can call roff_expand()
once again, which used to be called roff_res() before this

Rename mandoc_getarg() to roff_getarg() and pass it the roff parser
struct as an argument such that after copy-in, it can call roff_expand()
once again, which used to be called roff_res() before this. This
fixes a subtle low-level roff(7) parsing bug reported by Fabio
Scotoni <fabio at esse dot ch> in the 4.4BSD-Lite2 mdoc.samples(7)
manual page, because that page used an escaped escape sequence in
a macro argument.

To expand escaped escape sequences in quoted mdoc(7) arguments, too,
stop bypassing the call to roff_getarg() in mdoc_argv.c, function args()
for this case. This does not solve the case of escaped escape sequences
in quoted .Bl -column phrases yet.

Because roff_expand() can make the string longer, roff_getarg() can no
longer operate in-place but needs to malloc(3) the returned string.
In the high-level parsers, free(3) that string after processing it.

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