Use disable_sigint instead of sigatomic_(begin|end)

[tkf]

disable_sigint is the public API so we better use it instead of sigatomic_(begin|end).

A side effect is that it makes PyCall work with JuliaInterpreter.jl ouf-of-the-box and avoid issues like https://discourse.julialang.org/t/23560 and JuliaDebug/JuliaInterpreter.jl#219. This is because JuliaInterpreter.jl uses disable_sigint to workaround the issues related to the signal handler JuliaDebug/JuliaInterpreter.jl#108.

@stevengj Repeating what I asked in #683:

@stevengj You didn't want to put disable_sigint for all ccall #574 (comment) but maybe replacing only the sigatomic_begin-sigatomic_end pairs is OK?