Added wdir to devself and devproc:
- read '#0/wdir' to get the working directory of the calling process
NOTE that a read(fd, nil, -1) will return the negated length
of the working directory, just in case you want to
allocate the memory required
- read '/proc/n/wdir' to know the working directory of process n
(read(fd, nil, -1) still returns the negated length)
- write '#0/wdir' to change the working directory of the calling process
NOTE: no offset is allowed and the provided string must
be null terminated
- write '/proc/n/wdir' to change the working directory of process n
NOTE: no offset is allowed and the provided string must
be null terminated; moreover if another process change the working
directory change during the write, the current process will
receive an error.
In libc updated getwd() and chdir().
Also modified pwd to get advantage of the new file.
To test, run /arch/amd64/qa/kern/wdir.rc or simply try
% pwd
/usr/glenda
% echo -n /tmp > /proc/$pid/wdir
% pwd
/tmp
% cat '#0/wdir' && echo
/tmp
The expected use cases for wdir in devproc are rio and acme.
Also, note that we could theoretically remove the cd builtin
from rc and simply implement it as a rc function.
We don't do that to preserve rc portability to other OS.
