getrlimit and setrlimit interface

[Ludovic Courtès]

The ‘getrlimit’ and ‘setrlimit’ procedures are not documented in the
manual.  They’re supposed to be passed a symbol or a number:

  (getrlimit 'nofile) == (getrlimit 7) ≍ getrlimit (RLIMIT_NOFILE, &lim)

For most other POSIX functions with a similar interface, we instead
define Scheme variables with the same name as in C (like
‘RLIMIT_NOFILE’) and these are then passed directly to the procedure.

Here’s a proposal: define RLIMIT_* Scheme variables, document
‘getrlimit’ and ‘setrlimit’ to accept those, and emit a deprecation
warning when the symbol is used.

WDYT?

Ludo’.