C source cleanup

[ldoolitt]

Lint-free even with gcc warning flags turned up to 11

Only question is if y'all want the new Makefile lines
CFLAGS += -Wextra -Wstrict-prototypes -Wmissing-prototypes -Wshadow -pedantic
CFLAGS += -Wpointer-arith -Wcast-align -Wcast-qual -Wwrite-strings -Wundef
commented-out. This patch leaves them in.

[ldoolitt]

The changes in this PR are all in the category of clarifying intent to the compiler. They allow gcc to process the code -- even with the warning flags turned hyper-sensitive -- without emitting any warnings.
None of my changes will affect the run-time behavior of the code, so I can't claim to have fixed any bugs.
This is actually awesome! It shows that the original code is very clean.
I still recommend accepting my patches, since demonstrating that cleanliness (to gcc, and to the next programmer), and flagging any future regressions, has lasting value.

[liderbug]

Newer HS200 switches are using KLAP via port 80. Python-kasa works - although in my mind slow, 2+ seconds vs .002 seconds. Update? Upgrade? Thanks