Fix for dtostrf on floats under zero

Per forum
https://forum.pjrc.com/threads/1227-Teensy3-alternative-for-dtostrf()?p=86671&viewfull=1#post86671

Allow low frequencies for analogWriteFrequency

In case the desired frequency is too low to be obtained with F_TIMER
because prescale and mod were at their limits, switch automatically to
the alternative clock source (31250Hz) before determining prescale and
mod with the same algorithm as before.

SCAN Control register #define TSI_SCANC_EXTCHRG(n)

50.6.2 SCAN Control register

Bug found by Chronologist, https://forum.pjrc.com/threads/29624-Using-TSI-for-capacitance-measurement

https://forum.pjrc.com/threads/29446-Teensy-Hardware-Flow-Control-RTS-CTS?p=80425#post80425

Long datarefs for USB flight sim controls

Added full support for Midi Time Codes and Real Time messages

can now receive and send Midi Time Codes
and send Real Time messages

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1?p=76072&viewfull=1#post76072

Fix packet stuffing in usb_flightsim_flush_callback

https://forum.pjrc.com/threads/25153-PDB0CH1-register-definitions

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1?p=75775&viewfull=1#post75775

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1

LC TPM DMA Enable bit

https://forum.pjrc.com/threads/28325-Teensy-LC-and-delay

strlen is already in string.h

or, alternativly one could write size_t strlen(const char *s) {return __builtin_strlen(s);} to use the gcc-builtin function.
I think, both are faster than the bytewise search.

millis()

systick_millis_count is already volatile, doing it twice is not needed.