fix: Exchange support, file age calculation and safe arg quoting

[stephdl]

This PR fixes a connection issue observed with Exchange servers hosted at OVH, where imapsync was refusing to connect. Adding --addheader and --automap resolved the issue.

• Exchange compatibility: add --addheader and --automap to imapsync arguments. --addheader improves message deduplication by tagging already-synced messages; --automap handles folder name mapping differences common with Exchange servers (e.g. Sent Items vs Sent).
• File age calculation: read the previous folder sync value only when the timestamp file exists, avoiding silent failures in ash with set -e when the file was removed by a prior stop.
• Optional arg quoting: optional arguments (DELETE_LOCAL, DELETEFOLDERS, DELETE_REMOTE, EXPUNGE_REMOTE, SECURITY) now use the ${VAR:+"${VAR}"} pattern — the argument is passed quoted when the variable is set and non-empty, and omitted entirely when empty. This is POSIX/ash compatible and avoids passing empty strings to imapsync.

see https://community.nethserver.org/t/imapsync-exchange-anyone-else-fighting-with-this/27326