This reverts commit 4eb81994.

sys/unistd.h is not a standard header file, not found on musl system.
It is supposed to be unneeded.

only switch after the save of trees information, do not destroy the temporary db in case of crash: this will leave the running db in inconsistent state

adds field @extdepends. Services declared with @optsdepends are not mandatory. Services declared with @extdepends must have the frontend file installed at least

make a distinction between system error and unable to find the service, return -1 on system error, 0 for service error