init: Set and export PATH before running any external commands

busybox sh and klibc dash have different default values for $PATH
which could affect which could affect which commands we run later.

klibc dash also does not export $PATH, and this means we fail to
run fsck - it is executed indirectly through logsave, and logsave
cannot find it.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>