[9front] libmp and error handling

[covertusername967@gmail.com]

Hello,

I was working with libmp today trying to figure out error handling.
mp(2) says the following:

          Routines that return an mpint will allocate the mpint if the
          result parameter is nil.  This includes strtomp, itomp,
          uitomp, btomp, and dtomp. These functions, in addition to
          mpnew and mpcopy, will return nil if the allocation fails.

However, reading the source code reveals something entirely different:
while the routines do allocate a new mpint if needed, if allocation
fails they don't return nil and instead outright call sysfatal.  I
then checked the rest of the code in the system that uses libmp and
all of it relies on the undocumented behaviour, assuming that returned
pointers are valid.

There's both an easy way and a hard way to fix this.  The easy way
would be to document the sysfatal call in the manpage and mention that
sysfatal should be overridden in order to change the error handling.
The hard way would be to remove the sysfatal calls in libmp, then
change the rest of the system so that it catches the errors.

The hard way looks like it would be more correct, but could also break
compatibility with other software.  I don't know who uses libmp other
than the rest of userland, however, so I would like to ask if there
are any other users at all, and if so, if you object to changing
libmp.  I have a need for this and can put a patch together if this
change seems okay.