installation: allow profile modification to be skipped (#1072)

The current behaviour modifies the first writeable file from amongst
.bash_profile, .bash_login and .profile.  So .bash_profile (if it is
writable) would be modified even if a user has already sourced nix.sh
in, say, .profile.

This commit introduces a new environment variable,
NIX_INSTALLER_NO_MODIFY_PROFILE.  If this is set during installation,
then the modifications are unconditionally skipped.

This is useful for users who have a manually curated set of dotfiles
that they are porting to a new machine. In such scenarios, nix.sh is
already sourced at a place where the user prefers.  Without this
change, the nix installer would insist on modifying .bash_profile if
it exists.

This commit also add documentations for both the current behaviour and
the new override.