signal.c: avoid repeated disable/enable interrupt

We only install system-level signal handlers during init (before any
threads or user code is running), so there's no need to repeatedly
enable/disable the interrupts at that time.  We also do not change
system-level sig handlers once user-level code is running.

This saves about 20 syscalls at startup and makes the executable
smaller (numbers from 32-bit x86):

        text       data     bss     dec     hex filename
before: 2815726   12100   30552 2858378  2b9d8a ruby
 after: 2815022   12100   30552 2857674  2b9aca ruby

* signal.c (install_sighandler): remove rb_disable_interrupt and
  rb_enable_interrupt calls
  (init_sigchld): ditto
  (Init_signal): disable and enable interrupt once around all
  install_sighandler and init_sigchld to reduce syscalls at start
  [Feature #9345] [ruby-core:59480]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48178 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-10-29

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48179 b2dd03c8-39d4-4d8f-98ff-823fe69b080e