Update PWM interrupt comment, outdated since a1fb7afc1.
Remove confusing and likely-never-used "ultrapwm" ifdef section.

Holler if you actually used this.
Reintroduce MIN_RC_PULS to discard too-short pulses.

This really doesn't do anything other than avoid arming on things like
input from the USB linker, unless RC_PULS_REVERSE is set, which could
make such pulses look like full reverse throttle. However, slightly
longer but still short pulses (like those which may occur if the PWM
input wire or radio power or flight board power is yanked at a bad time)
may still look like full reverse throttle. We can only rely on a short
rc_timeout to stop things quickly in this case.
Clear obsolete comments.
Move reset code to the end.

This should be safer in the event of an interruped or incomplete flash,
and should allow for more interrupt space that can be reached by Z while
ZH is 0. For bigger chips, we might have to be careful about how far rjmp
can reach, but it's fine on the ATmega8 assuming wrapping is enabled.
Dynamically enable complementary PWM based on timing, split high side and
low side dead times, and use interrupts for long dead times (P/N boards).

Perhaps still not very useful on P/N boards, but we can do it now.
Add calibrated dead times for complementary PWM.
Add LOW_BRAKE to brake when input is below the calibrated low pulse.

This works well on my Airtronics XL2P with the thumb brake.
Fix boot loader self-update regression since 50ac1b7a3a.

Oops, sorry! You will have to use ISP to update if you used anything
between 50ac1b7a3a and before this commit.
Remove duplicate LED macros.