clonebundles: optional memory-requirement attribution
The new REQUIREDRAM option allows a client to skip bundles it isn't
expected to handle well, e.g. without swapping. This allows a fallback
path to be provided e.g. using zstd level 10 instead of 22.
Differential Revision: https://phab.mercurial-scm.org/D8645
util: provide a helper function to estimate RAM size
For POSIX systems, it uses sysconf. For Windows, it uses the win32 API
directly.
Differential Revision: https://phab.mercurial-scm.org/D8644