Bugfix in roff_setstrn():
Do not call strcmp() on an array of char that might not be NUL-terminated.
Major roff_getname() cleanup.
* Return the name even if it is terminated by an escape sequence, not a blank.
* Skip the full escape sequence using mandoc_escape(), not just the first byte.
* Make it non-destructive, return the length instead of writing a '\0'.
* Let .ds and .as cope with the above changes to the internal interface.
* Fix .rm and .rr to accept an escape sequence as the end of a name.
* Fix .nr and .rr to not set/delete a register with an empty name.