syscall: utility overhaul

syscall: utility overhaul

Following is a list of functional changes:

* The -o flag outputs the entire buffer to the length returned
  by the syscall, or, in case of fd2path(2) and errstr(2), to '\0'.
* The -x flag is removed; the above makes it possible to pipe
  into xd(1) to get the same result.
* The -s flag uses dirfmt(2) to format the stat message, instead
  of trying to imitate ls(1).
* Stderr reports are normalized and made easier to parse.

The code also suffered a number of stylistic changes.