libm: Add several new functions and symbol versioning

The following long double functions were added to the math library:
  logl
  log2l
  log10l
  log1pl
  expm1l
  acoshl
  asinhl
  atanhl

In addition, the FreeBSD functionality that creates symbol versioning
for libraries was adapted for FreeBSD.  The first version is called
"DFLY36.0".  If it is necessary to create a new version of the 3.5 or
3.6 branch, the number after the decimal will be incremented.  The 3.7
branch will start with "DFLY38.0" if it needs its own version.

libm was baselined with all symbols being the same version: DFLY36.0.
With symbol versioning, it will not be necessary to increment the major
version anymore, so this library shall always be known as libm.so.4 from
this point on.
Change initial symbol version from DFLY36.0 to DF306.0

The first one will make handling DragonFly 3.9 and later awkward.