Implement support for fallocate() call on Linux. 
Implement improvement CORE-4443 : Use fast file grow on those Linux systems which supports it

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@59642 65644016-39b1-43b1-bf79-96bc8fe82c15
Fixed bug CORE-4444 : Engine could hung and block all attachments in out of disk space condition during physical backup

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@59643 65644016-39b1-43b1-bf79-96bc8fe82c15
Fixed bug CORE-4444 : Engine could hung and block all attachments in out of disk space condition during physical backup
Improvement CORE-4445 : Extend main database file faster when physical backup state changed from stalled to merge

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@59644 65644016-39b1-43b1-bf79-96bc8fe82c15
Updated the docs.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@59646 65644016-39b1-43b1-bf79-96bc8fe82c15