Remove fishd.cpp source, docs, and target from Makefile and Xcode
project
Remove large parts of fishd interaction, including
env_universal_server
Removal of more fishd artifacts and headers
Remove connection_t usage from write_to_fd
Removed connection_t and associated functions
Teach env_universal_remove to directly report whether the variable was
successfully removed.
Fix Linux build of env_universal_common
Migrate global functions out of env_universal. Have env operate directly
on an env_universal_t.
Universal variable callbacks should only be announced for changed
values, not every value. Also support erase notifications.
Remove env_universal.h and env_universal.cpp from project
Merge branch 'death_of_fishd'

Incorporates more removal and cleanup of fishd remnants.