The getsecs() function is implemented in platform- and bootfw-specific
files and, in a number of these places, there were problems with how they
were declared.

Some used int return instead of time_t. On some architectures the bit
width of time_t did not naturally fit into an integer and could lead to
some unexpected behavior. (For example, 32-bit ARM builds uses a 64-bit
time_t.)

Make sure the function prototypes always specify void for the argument
list when they do not have any arguemnts, otherwise some compilers can
complain about the prototype.

Reported by: Kevin Zheng
Reviewed by: sjg
Approved by: sjg (mentor)
Obtained from: Juniper Networks, Inc.
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D7463