Fix JuliaLang/julia#6665

Provides airyx in addition to bessel[jyikh]x
Updates following discussion in JuliaLang/julia#6665

Rather than introducing bessel[hijky]x functions, introduce optional
positional argument to bessel[hijky] and airy for scaled version.
Merge branch 'master' of https://github.com/JuliaLang/julia into besselx

Conflicts:
	base/math.jl

Keeping track of special functions reorganization
fa742208b31a6640a82a6bf29f97c21cba471ff0
Revert back to bessel[ijkhy]x function signature

Change as discussed in JuliaLang/julia#6665.

Also fixed JuliaLang/julia#6564 that was omitted in this branch.
Merge pull request #6688 from jdrugo/besselx

Fix JuliaLang/julia#6665: adding exp-scaled Bessel functions