Do not leak 64 bytes of heap memory every time a manual page calls

Do not leak 64 bytes of heap memory every time a manual page calls
a user-defined macro.  Calls of standard mdoc(7) and man(7) macros
were unaffected, so the effect on OpenBSD manual pages was small,
about 80 Kilobytes grand total for a full run of "makewhatis
/usr/share/man".

Argument expansion contexts for user-defined macros are stored on
a stack that grows as needed if calls of user-defined macros are
nested or recursive.  Individual stack entries contain dynamically
allocated arrays of pointers to arguments; these argument arrays
also grow as needed if user-defined macros take more than eight
arguments.  The mistake was that argument arrays of already
initialized expansion contexts were leaked rather than reused on
subsequent macro calls.

I found this issue in a systematic hunt for memory leaks after
Michael <Stapelberg at Debian> reported memory exhaustion problems
on the production server manpages.debian.org.  This sub-Megabyte
leak is not the cause of Michael's trouble, though, where Gigabytes
of memory are being wasted.  We are still investigating whether the
original problem may be related to his supervisor process, which is
written in Go, rather than to mandoc.