Make "Checking how to use..." lower case.

The configure script generated by autoconf uses lower case "checking". Let's
keep in line. :)
completions for mosh (http://mosh.mit.edu)
Make create_manpage_completions not generate empty files
https://github.com/fish-shell/fish-shell/issues/506
Tweak test_fork to exercise strerror
Don't call strerror or perror after fork, it's unsafe. Added safe_strerror and safe_perror replacements.
Fixes https://github.com/fish-shell/fish-shell/issues/495
Fixed a minor typo: standrard -> standard
Fix an exception when parsing a multi-line command that ends in cd
https://github.com/fish-shell/fish-shell/issues/511
exec.cpp:exec: Don't remove block_io elements from j->io

When dulicate_prepend prepend copies of block_io elements to j->io, the
removing simply does nothing. Now that the same pointers are prepended to
j->io, don't remove them.
Revert "Revert shared_ptr<io_data_t> changes until kinks are ironed out"

This reverts commit 77f1b1f0fe27778750bb9b9aa53f6bc42d5e5843.
Make do_builtin_io multi-fork safe, moved it to postfork.cpp
Addresses https://github.com/fish-shell/fish-shell/issues/495