Provide retry options for upsdrvctl and driver(s)

As recently seen in Debian (bugs #694717 and #677143), it may be required to
have upsdrvctl retrying to start the driver in case of failure.  More
specifically, a mix of init system (V and systemd), udev and USB device(s) can
result in the /dev entry not being available at driver startup, thus resulting
in a general failure to start NUT.  This commit provides at least a way to
overcome this issue. A more suitable  solution will require more work on NUT
design.

This patch if based on Arnaud Quette proposal
configure.in: bump version to 2.7.1.5

Some packaging systems don't like the -pre# system.
Rename configure.in to configure.ac

autoconf has been warning about this for a while - let's fix it before too many
branches get created with the old name.
nutdrv_qx: add new 'fallback' Q1 subdriver

Add new 'Q1' subdriver.
This subdriver implements the same protocol as the one used by the 'megatec' subdriver minus the vendor (I) and ratings (F) queries.
In the claim function:
- it doesn't even try to get 'vendor' informations (I)
- it checks only status (Q1), through 'input.voltage' variable
Therefore it should be able to work even if the UPS doesn't support vendor/ratings *and* the user doesn't use the 'novendor'/'norating' flags, as long as:
- the UPS replies a Q1-compliant answer (i.e. not necessary filled with all of the Q1-required data, but at least of the right length and with not available data filled with
some replacement character)
- the UPS reports a valid input.voltage (used in the claim function)
- the UPS reports valid status bits (1st, 2nd, 3rd, 6th, 7th are the mandatory ones)

This commit reintroduces a functionality of the blazer subdrivers that was lost because now, in order to tell whether a device is supported by a subdriver or not, if the user
doesn't call the driver with the 'novendor' flag, both the status (Q1) and the vendor (I/FW?) queries are needed (that's to better discern the subdrivers).

Reference: http://lists.alioth.debian.org/pipermail/nut-upsuser/2013-November/008692.html
nutdrv_qx: update manuals for new Q1 subdriver and improve readability
nutdrv_qx: versioning
nutdrv_qx: improve developer manual

Get rid of useless tables.
Fix minor errors/typos.
nutdrv_qx: fix 'megatec/old' and 'mustek' subdrivers' claim functions

Address, for 'megatec/old' and 'mustek' subdrivers, the same problem fixed in commit 720975f4de910b270ba705a7f2981c2ee33ca2eb for Q1-based ones:
- Make the claim function of 'megatec/old' and 'mustek' subdrivers not poll the UPS for 'vendor' informations as they are not really needed to set these protocols
apart from the other ones (i.e. the 'status' poll is specific enough, at the time of writing).
- Move common 'light' claim function to nutdrv_qx_blazer-common.{c,h}.
- Update manual.
- Versioning.