Fix SEGV when $fgets, $sscanf, or $fscanf is used with string (#2604)

* Add a test to use string for $fgets

* Use dedicated function for $fgets to std::string

* share the implementation of $fgets

* Pass -1 for bitwidth of std::string to distinguish from POD

* add checks for scanf with string

* apply clang-format

test_regress/t/t_sys_file_basic_input.dat

@@ -1,10 +1,12 @@
 hi
 lquad
 widestuff
+string
 *xa=1f xb=237904689_02348923
 *ba=10 bb=11010010101001010101 note_the_two
 *oa=23 ob=12563
 *d=-236123
 *u=-236124
 *fredfishblah
+aBCD not_included
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