Implement appending to standard man(7) and mdoc(7) macros with .am.

With roff_getstrn(), provide finer control which definitions
can be used for what:
* All definitions can be used for .if d tests and .am appending.
* User-defined for \* expansion, .dei expansion, and macro calling.
* Predefined for \* expansion.
* Standard macros, original or renamed, for macro calling.

Several related improvements while here:
* Do not return string table entries that have explicitly been removed.
* Do not create a rentab entry when trying to rename a non-existent macro.
* Clear an existing rentab entry when the external interface
roff_setstr() is called with its name.
* Avoid trailing blanks in macro lines generated from renamed
and from aliased macros.
* Delete the duplicate __m*_reserved[] tables, just use roff_name[].

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