OST purge optimization: start purging data from the more used OST.

When checking OST usage and purging OSTs in their index order,
robinhood spent time purging no-so-full OSTs whereas
it should hurry purging very-full OSTs first.

This improvement consists in checking OST usage
in the order of their used pct or volume (depending
on the specified high threshold type).
logging: make log line header configurable.

Allow activating/deactivating information in log header:
- process name (specified by log::log_procname parameter)
- host name (specified by log::log_hostname parameter)
- module name (specified by log::log_module parameter)
listmgr: fix iterator filtering on both pool name and OST index.

Filtering a "select" request on both pool_name and ost_idx conditions
resulted in invalid request syntax (missing AND between 2 conditions).
rbh-find: added -pool option to filter entries by OST pool name.
rbh-find: added -exec and -print options.

Note that -exec command syntax differs from classical 'find'.
rbh-find: help update (-bulk option was missing).
rbh-find: added -nouser and -nogroup options.
rbh-find: added option -lsost to display OST information.
rbh-find: automatic switch to bulk mode when the argument is FS root.
fix non-reg tests for new logging and find parameters.
non-reg: added test 116 (test behavior when fs_path != mount point)
tests: added ost check order test (304) + fixed OST trigger tests
non-reg test fix: fs_path is not set for all tests.
backup: added non-reg test for compression.
backup: add non-reg test for disaster recovery with compression on/off.
hsm: fix size of recovered entries for compressed archives.
pkg: robinhood 2.5.1