Do not include headers inside __BEGIN_DECLS ... __END_DECLS blocks.

We don't have C++ unsafe headers (anymore).

I added a test to fix_headers.sh that checks if certain "unsafe"
headers are ONLY included inside a __BEGIN_DECLS ... __END_DECLS
(because after all, they are unsafe), as well as checking that
no other header files are included inside such a block. The rationale
of the latter is that if a file is a C header and it declares
function prototypes (otherwise it doesn't matter) and is sometimes
included outside a __BEGIN_DECLS ... __END_DECLS block (from a C++
source file) then it has to be C++ safe and doesn't ever to be
included from inside such a block; while if a file is a C++ header
then obviously it should never be included from such a block.

fix_headers.sh subsequently found several safe headers to be
included from inside such a block, and those that were (apparently
in the past) unsafe were included only sometimes inside such a
block and often outside it. I had a look at those files and saw
that at least an attempt has been made to make them C++ safe,
but especially because they already are included OFTEN outside
a __BEGIN_DECLS ... __END_DECLS (from C++ source files) the
best decision seems to treat them as safe.

This is not risky: .c files that define such functions still
generate C-linkage for their functions. If a C++ unsafe C header
is included outside a __BEGIN_DECLS ... __END_DECLS block then
the only possible result would be an undefined reference to
a function with C++-linkage that will not exist. Aka, when
something links after this commit, then the commit was correct.
I did build all targets and they all linked.

src/modules/systemlib/cpuload.h

@@ -35,8 +35,6 @@
 
 #ifdef CONFIG_SCHED_INSTRUMENTATION
 
-__BEGIN_DECLS
-
 #include <sched.h>
 #include <stdint.h>
 
@@ -58,6 +56,8 @@ struct system_load_s {
 	int sleeping_count;
 };
 
+__BEGIN_DECLS
+
 __EXPORT extern struct system_load_s system_load;
 
 __EXPORT void cpuload_initialize_once(void);

src/modules/systemlib/otp.h

@@ -43,8 +43,6 @@
 #ifndef OTP_H_
 #define OTP_H_
 
-__BEGIN_DECLS
-
 #define ADDR_OTP_START			0x1FFF7800
 #define ADDR_OTP_LOCK_START		0x1FFF7A00
 
@@ -130,6 +128,7 @@ union udid {
 };
 #pragma pack(pop)
 
+__BEGIN_DECLS
 
 /**
  *   s

src/modules/systemlib/printload.h

@@ -41,9 +41,8 @@
 
 #pragma once
 
-__BEGIN_DECLS
-
 #include <px4_config.h>
+
 #include <stdint.h>
 
 #ifndef CONFIG_MAX_TASKS
@@ -63,6 +62,8 @@ struct print_load_s {
 	float interval_time_ms_inv;
 };
 
+__BEGIN_DECLS
+
 __EXPORT void init_print_load_s(uint64_t t, struct print_load_s *s);
 
 __EXPORT void print_load(uint64_t t, int fd, struct print_load_s *print_state);