* Add middle click copy

* Make middle click action configurable

This introduces the NNN_MIDDLECLICK environment variable, containing the
key that middle mouse click emulates. Only the first character is read.

* Document NNN_MIDDLECLICK in nnn.1

* Use NNN_MCLICK instead of NNN_MIDDLECLICK

* Add '^key' syntax to set middle click to Ctrl+Key

Replaces the following environment variables:
- NNN_USE_EDITOR
- NNN_IDLE_TIMEOUT

This reverts commit 1ed5febe0c.

This reverts commit 278fcfdfc9.

- remove auto-stored session "@" on restore
- minor help update

This reverts commit cf388649b9.

- replace bools for internal non-persistent binary states with an uchar
- do not store path to session, derive from config path in tmp buffer
- program option to load session is not `-s`
- remove auto-stored session "@" on restore
- minor help update