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[].

1 files changed, 10 insertions, 0 deletions

diff --git a/regress/roff/string/std.in b/regress/roff/string/std.in
new file mode 100644
index 00000000..f460b101
--- /dev/null
+++ b/regress/roff/string/std.in
@@ -0,0 +1,10 @@
+.TH STRING-STD 1 "June 18, 2017" OpenBSD
+.SH NAME
+string-std - expanding standard macros as strings
+.SH DESCRIPTION
+expanding the DT macro as a string:
+>>>\*[DT]<<<
+.PP
+.rn DT myname
+the same after renaming it:
+>>>\*[myname]<<<