Decomposed agent install and reduced overhead in some cases.
Decomposed agent install and reduced overhead in some cases.

This commit decomposes the agent install step for POSIX remotes into
separate "chmod +x" and "install" steps. It also restricts usage of the
"chmod +x" step to cases where the agent binary was copied from a
Windows system, since that's the only place it can lose its
executability bit.

This change was necessary to support shells that don't support
"&&"-style command chaining (e.g. fish).

Fixes #26.

havoc-io/mutagen,havoc-io/mutagen