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, 15 insertions, 0 deletions

diff --git a/regress/roff/string/std.out_ascii b/regress/roff/string/std.out_ascii
new file mode 100644
index 00000000..b399374c
--- /dev/null
+++ b/regress/roff/string/std.out_ascii
@@ -0,0 +1,15 @@
+STRING-STD(1)               General Commands Manual              STRING-STD(1)
+
+
+
+NNAAMMEE
+       string-std - expanding standard macros as strings
+
+DDEESSCCRRIIPPTTIIOONN
+       expanding the DT macro as a string: >>><<<
+
+       the same after renaming it: >>><<<
+
+
+
+OpenBSD                          June 18, 2017                   STRING-STD(1)